Ronaldo's return is only the start. When Real Madrid and Manchester United meet in the UEFA Champions League's Round of 16 early next year, the two-legged tie could—maybe even should—become a classic.

Why? That's easy—just look at the history, the matches, the personal stories, the sub-plots, the form book and even the managers. Nearly every bit of information about this matchup provides intrigue.

On that much, we can all agree. What's less certain is who will win. Fortunately, a deeper look at the intrigue provides a hint or two.

Let's start with the obvious.

Cristiano Ronaldo is returning to Old Trafford to face Manchester United, his club home from 2003 to 2009. When he joined the club, he was a promising, immensely talented teenager with buckets of potential. When he left for a world-record £80 million six years later, he was a bona fide superstar.

Ronaldo still considers Sir Alex Ferguson, Manchester United's manager, his "father in football" (via The Independent). That doesn't mean he'll take it easy on the venerable Scot. On the contrary, his return could serve to motivate, and he will surely relish the chance to prove himself against his former mentor.

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Not that he's the only one with a point to prove.

Manchester United crashed out of the competition at the group stage last year. Everyone at the club, from Ferguson to attacking superstar Wayne Rooney, remembers. Rooney and United won a European title in 2008 with Ronaldo in the team, and this could be their chance to show they no longer need Ronaldo's brilliance.

With the addition of Robin van Persie to an already potent attack, United's offensive prowess now qualifies as truly fearsome. Most of their problems, however, have occurred at the other end of the pitch this season. Coincidentally, so have Real's.

United's defense has shipped an alarming 24 goals in 17 Premier League matches this season. Struck by a series of injuries, the unit has struggled at times, conceding multiple goals in eight league matches. By comparison, they have kept just three clean sheets.

Real Madrid have endured their own defensive struggles, conceding twice in both of their last two La Liga matches. Their record of 14 goals conceded in 16 league matches is considerably better than United's, and Real's defense and defensive midfielder are both better than United's on paper.

But United have captain and central defender Nemanja Vidic returning to the squad after a long injury layoff. He could give United a decisive boost.

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Much will depend on the managers, of course, and both rank among the top five or so in the world. Ferguson and Jose Mourinho produced a series of epic, memorable battles while Mourinho managed Chelsea, and this two-legged tie should be no different.

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United currently sit atop the Premier League while Real trail rivals Barcelona by a seemingly insurmountable margin in La Liga. That could mean Real put all their focus on this tie—or that United are clear favorites.

As the match approaches, we should be able to expect both managers to make the right decisions and have their teams ready. Both know the history, as we all do: Real have won a record nine European Cup/Champions League titles, United three. The winner of this tie could be a decent bet to add another to their resume this season.

And if the history between these clubs is any indication, we should expect a rollicking, dramatic, tension-filled two-legged tie.